DACs and analog-to-digital converters (ADCs) have come recently into more widespread use. This is due in part to improvements in process and implementation technologies as well as an increase in applications for such converters. Applications include use in multimedia, audio, video and computer peripheral products, for examples.
One common type of DAC is the so-called "string DAC". A typical N-bit string DAC, used for converting an N-bit digital word to an analog voltage, includes 2.sup.N taps to a resistor ladder. The resistor ladder includes 2.sup.N resistors connected in series between positive and negative supply voltages. The taps to the resistor ladder are connected through switches to an output of the DAC. Each adjacent set of taps is separated by a different one of the resistors within the ladder.
During operation, the N-bit digital word controls the switches such that one and only one of the switches closes connecting one of the taps to the DAC output. The voltage appearing at that tap is provided as an output of the DAC. As will be understood, the resistor ladder acts as a large voltage divider with each tap of the ladder being at a different voltage value. The tap selected for connection to the output depends on the digital word input to the DAC. Typically, the digital input word is decoded for controlling the switches to close one. Often, as the digital input word increases, so does the analog output voltage.
String DACs are fairly simple to implement and are inherently monotonic. String DACs are used commonly for particular applications. For example, string DACs often are used to provide reference voltage output taps in flash ADCs. Similarly, string DACs are used in applications where rail-to-rail outputs are required and a load need not be driven.
One drawback associated with string DACs is their size requirement. Typically, for an N-bit string DAC, 2.sup.N taps (and resistors) are required. For a significant number of bits, i.e.,bits and greater, the size can become a significant factor, particularly where large resistors are required for realizing small resistances.